The invention relates firstly to a process for producing a component for reducing the effect of an external mechanical impact, of the type which comprises an essentially tubular metal element which has a jump in its cross section which is such that under the influence of an axial load it can be turned at least partially inside-out as a result of plastic deformation. The invention also relates to components produced in this way.
There is a general need in the engineering sector for components which are able to reduce the effect of an external mechanical impact. A typical example is the automotive industry.
As traffic becomes ever heavier, many cars are subject to collisions, which may lead to considerable damage and harm to vehicles and people. For this reason, it is generally thought to provide these vehicles with components which are able to reduce the damage caused by collisions. It is generally known to protect the passenger compartment of the vehicle by means of so-called crumple zones at the front and rear of the vehicle. However, considerable material damage may still occur and can only be limited by absorbing part of the collision energy in or immediately next to the bumper. For this reason, considerable research has been devoted to the production of components which are coupled to the bumper and in which some of the collision energy can be dissipated as a result of plastic deformation of an element.
It should be noted, however, that the need for components for reducing the effect of an external mechanical impact is not limited to automotive engineering.
British Patent No. 884,953 describes a component for dissipating collision energy between the bumper and the chassis of an automobile, which component comprises a tubular element of the type described in the preamble. During a collision, the outside of a piece of tube is turned inwards, thus converting collision energy into energy required to achieve the plastic deformation of the tubular element.
It should be noted that the tubular element forms part of a component that is to be built into the overall structure of an automobile or of another mechanical structure, specifically in such a manner that the forces resulting from the external impact can be transmitted to and absorbed by the tubular element. This complete component is known to those skilled in the art by the term xe2x80x9ccrash unitxe2x80x9d. The tubular element which can be turned at least partially inside-out is also referred to in technical language by the term xe2x80x9cinvertubexe2x80x9d and the movement by which it is turned inside-out is also known as xe2x80x9cinversionxe2x80x9d. These terms will be used in the following text.
It is generally desired to make crash units as lightweight as possible using inexpensive production methods. U.S. Pat. No. 3,599,757 describes how an xe2x80x9cinvertubexe2x80x9d is formed in three steps. Firstly, a piece of tube is expanded by hydroforming. Then, a reinforcement ring is attached at the location of a transition which has been formed, after which this unit is deformed under axial pressure.
The object of the invention is therefore to provide a simple and inexpensive production method, by means of which the finished invertube can be produced in a single operation without using any auxiliary means.
The invention consists in the fact that the tubular element is formed by hydroforming from a piece of tube which has a cross section which is constant along its length, during which process part of the length of the cross section is increased in size, and the transitions in the cross section cover a short distance, and during the hydroforming an axial pressure on the piece of tube which is being deformed is set to be such that the transitions in cross section adopt an S-shape.
The technique of hydroforming is extensively described in the specialist literature and does not require any further explanation here.
It has been found that using this new production method a piece of tube can be formed into an invertube which is suitable for a crash unit in a single operation, at reduced cost and without employed additional means.
The shape of an invertube in which the transition is S-shaped in cross section is known from German Laid-Open Specification DE 4316164. This makes it easier to initiate the inversion of the invertube when it is subjected to axial load, with the result that a regular, desired deformation of the invertube is obtained.
It has now been found according to the invention that such a shape can also be obtained using the hydroforming technique as a result of setting an axial pressure on the piece of tube which is being deformed during hydroforming to be such that the transitions in cross section adopt an S-shape. It should be noted that normally, during hydroforming, the expanding tube nestles against die halves. However, it has been found that using a higher axial pressure makes it possible to shorten the deformed piece of tube axially, so that the transitions come away from the die and adopt an S-shape.
The tubular element formed has straight tube ends at its extremities. These can be clamped directly into attachment sleeves which respectively form part of the bumper and the chassis of the car. However, it is also possible, according to the invention, to provide the tubular element itself with attachment means so as to form a crash unit. Attachment means of this nature may comprise flanges formed by turning down the ends of the element.
The most simple shape which can be obtained by hydroforming is a double invertube, which is in fact two invertubes which follow one another.
If a single invertube of the type described, for example, in the above mentioned German Laid-Open Specification is preferred, such a tubular element can be produced by cutting through the piece of tube which has been deformed by hydroforming halfway along its length.
In addition to the process described, the invention also relates to a component which is suitable for reducing the effect of an external mechanical impact, of the type which comprises an essentially tubular metal element which has a jump in its cross section which is such that, under the influence of an axial load, it can be turned at least partially inside-out as a result of plastic deformation the transitions in cross section of the tubular element being S-shaped. A component of this nature may then be obtained by using the process described above.
In order to incorporate the tubular element to form a new embodiment of the component, this tubular element may be provided with attachment means. These attachment means may comprise flanges which are formed by turning down the ends of the element.
Furthermore, a new design of the crash unit according to the invention is formed by providing the tubular metal element with a plurality of transitions in cross section. Finally, according to the invention the new component may form a single unit with a longitudinal beam of a motor vehicle. If this longitudinal beam is also produced using the hydroforming technique from a tube, it is possible to produce the invertube in a single production step together with such a longitudinal beam.